这是我的问题,经过一些研究,并尝试了,我的应用程序FC,当我有以下行(由*指定)

private TextView msg;

msg = (TextView) findViewById(R.id.txtviewOut) ;

*    msg.setVisibility(View.INVISIBLE);

我不知道为什么

这是我所有代码的其余部分：

main.xml中

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context=".MainActivity$PlaceholderFragment">

    <EditText
        android:id="@+id/textenter"
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:layout_below="@+id/lbledt1"
        android:layout_alignParentLeft="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Enter Name"
        android:id="@+id/lbledt1"
        android:layout_marginTop="26dp"
        android:layout_centerHorizontal="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Enter your date of birth (e.g. xx July 19xx)"
        android:id="@+id/textView"
        android:layout_below="@+id/textenter"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="57dp" />

    <EditText
        android:id="@+id/editText"
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:layout_below="@+id/textView"
        android:layout_alignParentLeft="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Press this button"
        android:id="@+id/button"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="You have been awesome since"
        android:id="@+id/txtviewOut"
        android:layout_marginTop="86dp"
        android:textIsSelectable="false"
        android:visibility="invisible"
        android:layout_below="@+id/button"
        android:layout_centerHorizontal="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:id="@+id/txtoutName"
        android:textIsSelectable="false"
        android:layout_alignBottom="@+id/txtviewOut"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="41dp" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:id="@+id/txtOutDate"
        android:layout_marginTop="28dp"
        android:textIsSelectable="false"
        android:layout_below="@+id/txtviewOut"
        android:layout_centerHorizontal="true" />

</RelativeLayout>

这是来自MainActivity.java

package com.example.helloandroidstudio;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

    private Button btnClick;
    private EditText Name,Date;
    private TextView msg,NameOut,DateOut;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btnClick = (Button) findViewById(R.id.button) ;
        btnClick.setOnClickListener(this);
        Name = (EditText) findViewById(R.id.textenter) ;
        Date = (EditText) findViewById(R.id.editText) ;
        msg = (TextView) findViewById(R.id.txtviewOut) ;
        msg.setVisibility(View.INVISIBLE);
        NameOut = (TextView) findViewById(R.id.txtoutName) ;
        DateOut = (TextView) findViewById(R.id.txtOutDate) ;
        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container,new PlaceholderFragment())
                    .commit();
        }
    }

    public void onClick(View v)
    {
        if (v == btnClick)
        {
            if (Name.equals("") == false && Date.equals("") == false)
            {
                NameOut = Name;
                DateOut = Date;
                msg.setVisibility(View.VISIBLE);
            }
            else
            {
                msg.setText("Please complete both fields");
                msg.setVisibility(View.VISIBLE);
            }
        }
        return;

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main,menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button,so long
        // as you specify a parent activity in AndroidManifest.xml.
        switch (item.getItemId()) {
            case R.id.action_settings:
                return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main,container,false);
            return rootView;
        }
    }

}

任何援助将不胜感激